We have performed O self-diffusion experiments in synthetic diopside single crystals along the b-axis at temperatures ranging from 1473-1643 K, under controlled O partial pressure (10 (super -11) -10 (super -2) atm). The 18 O tracer diffusion was imposed by solid/gas exchange between 16 O in diopside and 18 O 2 -enriched argon-hydrogen-H 2 O gas mixture. Diffusion profiles of 18 O were measured by Nuclear Reaction Analysis 18 O (p,alpha ) 15 N. The diffusion coefficients are described by D = D 0 exp (-E/RT), with log D 0 (m 2 /s) = -9.2+ or -1.0 and E = 310+ or -30 kJ/mol. Our results are in agreement with Ryerson and McKeegan's (1994) data and Farver's (1989) data along a direction perpendicular to the c direction. Experiments performed in a wide pO 2 range show that D is independent of pO 2 . We observe no change in the diffusion regime up to 1643 K (i.e. 22 K prior to melting temperature). This result differs from the diffusion study of Ca in diopside by Dimanov and Ingrin (1995), where a strong enhancement of Ca mobility, attributed to an excess disorder in the Ca-sublattice, was observed above 1523 K. We conclude that O diffusion in diopside is not affected by this premelting phenomenon.

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